Electrodeless lamps with which the present invention is concerned generally comprise a lamp envelope containing a plasma-forming medium. The medium in the envelope is excited with microwave, RF or other electromagnetic energy, thereby generating a plasma which emits radiation in the ultraviolet, visible or infrared part of the spectrum. Important uses of such electrodeless lamps are in connection with semiconductor manufacturing operations and for curing coatings or inks by photopolymerization reactions.
An electrodeless lamp transfers large amounts of heat to the lamp envelope during operation, and it has been found that the effectiveness with which the lamp envelope is cooled limits the overall lamp performance. Specifically, the brightness with which energy is radiated by the lamp increases as the power density of the microwave or other energy in the lamp envelope increases. However, as the power density increases, the envelope temperature increases, eventually reaching a temperature at which the envelope melts if not adequately cooled. Thus, the brightness which can be obtained from a lamp is dependent upon the ability to cool the lamp envelope. Moreover, even where a lamp operates satisfactory at a given envelope temperature, further cooling of the envelope to a lower temperature substantially increases the life of the bulb.
Several apparatuses for cooling electrodeless lamps are known, examples of which are is disclosed in U.S. Pat. Nos. 5,021,704, 4,485,332, and 4,042,850. These known methods include techniques certain extent, and for achieving a uniform temperature profile on the surface of the envelope. However, operation at still higher powers require a better approach to cooling.
Prior art cooling devices use common convergent outlet nozzles on cooling conduits which provide cooling air flow to the lamp envelope. In convergent cooling conduit nozzles, the nozzle exit is the smallest section through which gas flows. In this type of nozzle, gas flows from a large diameter entry portion, through a tapered section of the conduit nozzle where the flow area of the nozzle is steadily diminished until the gas reaches the nozzle exit. With convergent nozzles, the maximum velocity that can be obtained is the sonic velocity at the nozzle exit. This maximum velocity is obtained as the pressure at the nozzle exit approaches that of a critical pressure of the particular gas. If the pressure at the nozzle exit is greater than the critical pressure of the gas, the exit velocity is less than sonic velocity. Raising the exit pressure beyond the critical pressure of the gas will not increase the exit velocity. Also, if the exit pressure is substantially below that of the critical pressure of the gas, a sudden uncontrolled expansion will occur at the nozzle exit causing a disruption in the smooth jet flow, and consequently, lesser air flow velocity to the lamp envelope.
Existing electrodeless lamps use a plurality of cooling conduits extending from and secured to a cavity wall of the electrodeless lamp. Cooling air flows through these conduits and outlet nozzles in the conduits to the surface of the lamp envelope as the envelope rotates. The cooling conduits typically are deployed in a circumferential pattern around the lamp envelope. For better and more uniform cooling, it is desirable to position the conduit nozzles surrounding the lamp envelope in different horizontal planes to provide air to the lamp envelope at several elevations with respect to the envelope.
In order to provide varying elevational locations for the cooling conduit outlet nozzles, existing cooling conduits for such lamps have been designed with varying lengths, typically three standard sizes. These variable size conduits are necessary to achieve the differing elevational locations of the nozzles because the design of existing cooling systems are limited to securing the cooling conduits to the cavity wall in a common horizontal plane with respect to each other.
This existing design of cooling conduits has several disadvantages. Lamp envelopes of these lamps are attached via their own conduit extending upward to a manifold. The lamp envelope is vertically adjustable for optimizing coupling of the microwave field. However, since the cooling conduits are fixed to the cavity walls of the lamp, when the vertical position of the envelope is changed, the cooling dynamics of the envelope change due to a change in the relative position of the cooling conduit outlet nozzles with respect to the lamp envelope.
Further, cooling conduits for prior art electrodeless lamps have to be available in several different lengths to accommodate the need to have air exiting different conduit nozzles at different elevations relative to the lamp envelope. Having varying length cooling conduits increases costs associated with producing electrodeless lamps as well as costs associated with providing spare parts for the lamps. Even more troublesome is the problem, when replacing cooling conduits, of assuring that a proper length conduit is installed in a given position. Installation of a wrong combination of conduit lengths causes overheating and melting of the lamp envelope, resulting in lamp failure.